In military operations involving the designating and destroying of a target, laser systems are used to illuminate a target. This light is reradiated in all directions by the target and is used by a missile for homing in on the target. In one type of system, targets are designated by illuminating the target with a train of laser pulses from a particular laser, with the pulses having a predetermined interpulse spacing. A typical method of countermeasuring this laser target illumination is by detecting the pulses at the target, determining their pulse repetition frequency (PRF) or pulse interval modulation (PIM) and producing like laser pulses which are aimed at a reflective object removed from the target.
In an effort to prevent the countermeasuring of the illuminating laser, the subject invention involves a new type laser which is modulated to produce pulse pairs or doublets which cannot be easily detected and reproduced by the target. In order to be effective the interpulse spacing between the pulses of the doublet must be small and the spacing must be very accurate since it is extremely difficult to detect the fact that the laser radiation is pulsed when pulse spacings are very small. Moreover, even if the double laser pulses are detected, it is extremely difficult to generate like laser pulses because it is difficult to duplicate exactly the small interpulse spacings. Therefore the combatants who have apparatus for producing double pulse laser beams with extremely short accurately controlled interpulse spacings will have a virtually uncountermeasurable system.
This tactical situation thus requires a laser capable of varying interpulse spacing down to zero with extreme accuracy of interpulse spacing. Moreover, this tactical situation requires that pulse spacing be easily varied so that the illuminating laser can quickly change its output waveform to avoid detection.
Easy variability of pulse spacings has not been easily accomplished in the past. Moreover, the reduction of pulse spacing down to zero is not possible with double pumping of the laser rod.
In the prior art, techniques for obtaining multiple output pulses can be divided into two classes; (1) systems in which the separation between pulses is directly proportional to the time required to restore the population inversion in the lasing medium and (2) those system which are independent of the restoration time of the lasing medium. Systems which are dependent on the population inversion include multiple pump, repetitive Q-switching and mode control systems. An example of the multiple pump system is illustrated in U.S. Pat. No. 3,783,403. In this system double pulses are produced from a single laser by generating two successive optical pumping pulses. However, pulse spacing cannot be decreased to zero in this system.
An example of a laser system which is independent of the restoration time is described in an article by M. J. Landry, entitled "Variably Spaced Giant Pulses for Multiple Laser Cavities In a Single Lasing Medium", Applied Physics Letter, Vol. 18, pages 494-496. In this method, a prism is used to effectively split the laser rod longitudinally into two rods. This requires a rod of relatively large cross section. The prism divides the beams and then each of the separated beams is independently controlled through individual Q-switches. However, in the Landry system only one wavelength or color is utilized and the stress birefringent effect to be described hereinafter is not used.
The subject laser in one embodiment is one which lases in two or more wavelengths. The colors result from radiative transitions which can occur in two different ways: (1)from a single high energy level to multiple different lower energy levels or (2)from multiple different upper energy levels to a single low energy level. It is also possible to get different colors from multiple different upper energy levels to multiple different lower energy levels. This situation will be discussed later in connection with Er:YLF lasers. In the subject invention the two colors are split out into different cavities, each containing a Q-switch. The Q-switches are activated sequentially with a predetermined delay such that light of one color is coupled back through the laser rod to produce a first pulse, while the activation of the second Q-switch couples light of the other color back through the laser rod to produce a second pulse. Pulse spacing is controlled by the delay in activation of the second Q-switch. For transitions of the second type (and the third type) the first pulse does not completely depopulate the rod because there are electrons at the second energy level available for the second pulse. This sytem is useable with ruby rods and some Nd:YLF rods.
When, however, there are two transitions both starting from the same excited level, stress birefringence induced by the pumping of the rod may play a major part in permitting the production of two pulses. When two transitions start from the same excited level, it is possible that the production of the first pulse will completely depopulate the laser rod making two pulse production impossible. In some instances this can be prevented by very rapid Q-switching. However, stress birefringence provides simpler, more effective way of preventing complete depopulation during the production of the first pulse.
Stress birefringence occurs because some of the energy pumped into the laser rod is converted to heat which changes the characteristics of the rod such that two beams are produced each having a different polarization. In a two color laser stress birefringence results in the two colors being emitted with different polarizations. When the beams are separated into two differently polarized beams by a polarizing beam splitter then one polarized beam only partially depopulates the rod for the first pulse, thereby leaving enough of a population inversion for the formation of the second pulse. The Q-switching of the second cavity then can couple the remaining energy out of the rod to produce the second pulse.
It is a finding of the present invention that stress birefringence when the laser is pumped produces beams having orthogonal polarizations. The beams are separated according to polarization and each are Q-switched at different times to produce multiple pulses. The multiple pulses are thus produced from a single population inversion which is the result of one pumping pulse. Each polarity beam selectively depopulates the laser host material so that only a portion of the rod population is depopulated with each polarization to enable the production of two pulses when the Q-switching turn on is not rapid enough.
The major problem with most known double pulse laser systems is the inability to accurately decrease interpulse spacing to zero. While, as explained later, very small pulse spacings may not be necessary in such fields as double exposure holography, the ability to achieve very small pulse spacings is extremely important in the fields of communications for increased bandwidth and in military target designation, where laser beams having extremely small pulse spacings are required.
With respect to applications of multiple pulse lasers, as stated in the above mentioned patent the double pulse output may be used in double exposure holography in which two holograms are formed in rapid succession to detect any small disturbance occuring in the time between the two pulses. Double exposure holography is described in U.S. Pat. No. 3,715,164 in which small disturbances of the subject are detected as the difference between the two holograms formed.
In the case of double exposure holography the two pulses are responsible for forming two successive holograms, with the pulse spacing providing the time interval between the two exposures. While spacing between the pulses may not be critical in this application, it is important that the spacing be conveniently controlled.
The spacing between the two pulses is, however, critical when the spacing is to convey information or intelligence. Intelligence can be conveyed by the laser beam in the form of the coding of pulses in a pulse train produced when the laser is turned on and off in a predetermined manner. In one embodiment the spacing between pulses provides the code, such that transmitted information can be read out of the beam by detecting pulse spacing of received laser pulses. For instance, an aircraft can convey its altitude to a ground station over a laser-powered optical link, with the pulse spacing indicating altitude or any other desired parameter.
Moreover, in laser optical communications, when pulse spacing is used as the modulation parameter, it is important that pulse spacing be easily controlled and varied down to zero for the packing of more information into a given time interval.
Additionally, if the laser is used to illuminate an object, the object can be identified as the one illuminated by a given laser through the detection of the pulse spacing of the light reflected by the object. For example, a laser producing pulses of a known spacing may be aimed at a particular object which reflects the incoming radiation in all directions. If radiation from an object has this known pulse spacing then it is known that this object is the one at which the laser is aimed. Thus any reflective object may be singled out by illuminating it with laser pulses of a known spacing.
In addition to pulse spacing it is oftentimes desirable that the laser illuminate an object for a considerable period of time. If one of the multiple pulses is elongated then, in addition to pulse spacing, a multiple pulse laser can be used for illumination as well as the transfer of information.
These two aspects, e. g. minimum pulse spacing and pulse elongation, are important when in a military environment a target is to be both illuminated by a laser and identified by pulse spacing. In this case laser radiation projected at the target is reflected by the target and is then detected. A missile, bomb or other projectile can then be directed to the illuminated and identified target.
Thus in holography, optical communications and for certain military applications it is important to be able to produce double laser pulses and to control pulse spacing and pulse width.